1. Technical Field
This invention relates to chemical analysis, and more particularly systems combining an ion trap array with a spectrometer for chemical analysis.
2. Description of the Related Art
Time-of-flight (TOF) mass spectrometry is an analytical technique that is widely used because of its simplicity and wide mass range. In an idealized TOF system, ions are initially confined to a small spatial region and are nearly at rest near an electrode. However, in real TOF-based systems, the ions are initially neither nearly at rest nor in a well defined spatial region.
At certain discrete times, generally denoted as t=0, the ions are accelerated by an applied electric field imposed between an acceleration grid and an electrode sheet where the ions initially reside. The ions are then allowed to drift in a zero field region located between the acceleration grid and a detector until they reach the detector. The arrival time of the ions can be related to their mass because the heavier ions achieve a lower velocity while in the acceleration zone as compared to lighter ions. Thus, the method requires that the ions be pulsed in time or in a beam that is chopped at high frequency. There are many configurations of time-of-flight mass spectrometers. For example, some use reflection of the ions in an attempt to compensate for different initial velocities at the start of the acceleration that would otherwise significantly reduce the mass resolution.
The mass resolution of a TOF mass spectrometer depends on the ability to measure the drift time of ions with high precision. One way to achieve this precision is to ensure that all ions have low initial velocities and are spatially localized in a small region at the initial time. An ion trap can be used to achieve this initial condition by trapping and cooling sample ions until the initial time, at which time all ions are released together. Cooling the ions lowers the velocity of the ions. An additional advantage is that ions can be accumulated in the trap between extraction pulses so that the number of ions detected at a given time will be higher, thus increasing sensitivity.
Ion mobility spectrometry (IMS) is another form of chemical analysis that is similar to TOF mass spectrometry, but identifies chemical species based on drift time through a drift channel. The mechanical arrangement for IMS is about the same as in TOF. Ions start at t=0 in a confined region, then are allowed to drift through a constant field region to a detector, with an arrival time inversely proportional to the ion mobility. As with TOF, measurement resolution is improved by spatially localizing the ions in a small region at the initial time.
IMS is performed at higher pressure, even atmospheric pressure, versus a high vacuum for TOF-mass spectrometry. The gas that is present in IMS causes a viscous drag on the ions so it is necessary to have an electric field in the drift region. In practice, the drift and acceleration regions are generally merged into one drift channel. The ions move through the drift region with a velocity that is proportional to the electric field. The proportionality constant is characteristic of the ion but not quite as informative as the mass. Also, the resolution is degraded because of the diffusion that takes place during the drift.
In addition, in IMS the ion velocity is proportional to the applied field, whereas in TOF-mass spectrometry the ion acceleration is proportional to the applied field. IMS has a wide variety of applications currently because it does not require a vacuum system and is the method generally used in airports to test baggage for explosives and drugs, and also by the military for CW detection.
Ramsey et al., U.S. Pat. No. 6,469,298, includes common inventors to the present invention and describes an ion trap for mass spectrometry chemical analysis in which the ion trap is a single submillimeter trap. Ramsey '298 is hereby incorporated by reference in its entirety in the current application.
FIG. 1 illustrates a micro ion trap 10 disclosed by Ramsey '298. A ring electrode 12 is formed by producing a centrally located hole of appropriate diameter in a plate of a suitable material, such as stainless steel. In one aspect, the hole's radius r0 is 0.5 mm, so the diameter of the drilled hole in ring electrode 12 is 1.0 mm.
Planar end caps 14 and 16 generally comprise either stainless steel sheets or mesh, although other electrically conductive materials generally comprising metals or metal alloys may be used. The end caps 14 and 16 include a centrally located recess which can have a diameter on first dimension and a bottom surface of the recess having a hole of a second, lesser dimension. End caps 14 and 16 are separated from ring electrode 12 by insulators 18 and 20, each of which include a centrally located hole. Insulators 18 and 20 may comprise any suitable material, such as polytetrafluoroethylene sheet.
The holes in the ring electrode 12, end caps 14 and 16, and insulators 18 and 20 can be produced using conventional machining techniques. However, the holes could be formed using other methods such as wet chemical etching, plasma etching, or laser machining. Moreover, the conductive materials employed for ring electrode 12, and end caps 14 and 16 could be other than described above. For example, the conductive materials used could be various other metals, or doped semiconductor material. Similarly, polytetrafluoroethylene sheet need not necessarily be the material of choice for insulators 18 and 20. Insulators 18 and 20 could be formed from other plastics, ceramics, or glasses including thin films of such materials on the conductive materials.
The centrally located holes in ring electrode 12, end caps 14 and 16, and insulators 18 and 20 are preferably coaxially and symmetrically aligned about a vertical axis (not shown), to permit entry of ions from an external ion source or a structure within the trap to generate ions within the trap and permit ion ejection. When assembled into a sandwich construction, the interior surfaces of micro ion trap 10 form a generally tubular shape, and bound a partially enclosed cavity with a corresponding cylindrical shape.
In a preferred embodiment disclosed by Ramsey '298, micro ion trap 10 is a submillimeter trap having a cavity with: 1) an effective length 2z0 with z0 less than 1.0 mm; 2) an effective radius r0 less than 1.0 mm; and 3) a z0/r0 ratio greater than 0.83. However, z0 and/or an r0 greater than or equal to 1.0 mm could be employed while maintaining a z0/r0 ratio greater than 0.83. Although Ramsey '298 provides improved mass resolution and a smaller ion trap compared to conventional traps, higher storage capacity, improved mass resolution and greater sensitivity would be desirable.